Post-processing apparatuses to perform post processing of sheets, such as, aligning, sorting, stapling, punching, and folding of sheets are widely used and are often disposed downstream from an image forming apparatus to perform post-processing of the sheets output from the image forming apparatus. At present, post-processing apparatuses generally perform saddle-stitching, that is, stitching or stapling a bundle of sheets along its centerline, in addition to conventional edge-stitching along an edge portion of sheets. Therefore, to improve the quality of output sheets, several approaches to shape the folded portion of a bundle of saddle-stitched sheets have been proposed.
More specifically, when a bundle of sheets is saddle-stitched or saddle-stapled and then folded in two (hereinafter “booklet”), the folded portion around its spine tends to bulge, degrading the overall appearance of the booklet. In addition, because the bulging spine makes the booklet thicker on the spine side and thinner on the opposite side, when the booklets are piled together with the bulging spines on the same side, the piled booklets tilt more as the number of the booklets increases. Consequently, the booklets might fall over when piled together.
Therefore, in saddle-stitching or saddle-stapling, which is widely used as a simple bookbinding method, it is preferred to reduce bulging of the spine of the bundle of sheets thus bound, that is, to flatten the spine of the booklet. When the spine of the booklet is flattened, bulging of the booklet can be reduced, and accordingly multiple booklets can be piled together. This flattening is important for ease of storage and transport because it is difficult to stack booklets together if their spines bulge, making it difficult to store or carry them. With this reformation, relatively large number of booklets can be piled together. It is to be noted that the term “spine” used herein means not only the stitched or stapled side of the booklet but also portions of the front cover and the back cover continuous with the spine.
For example, in JP-2001-260564-A, the spine of the booklet is flattened using a pressing member configured to clamp simultaneously, from a front cover side and a back cover side of the booklet, an end portion of the booklet adjacent to the spine, and a spine forming roller configured to roll along the spine longitudinally. The spine forming roller moves at least once over the entire length of the spine of the booklet being fixed by the pressing member while applying to the spine a pressure sufficient to flatten the spine. Hereinafter the above-described mechanism is referred to as a spine formation mechanism.
Although this approach can flatten the spine of the booklet to a certain extent, it is possible that the sheets might wrinkle and be torn around the spine or folded portion because the pressure roller applies localized pressure to the spine continuously. In addition, although generally not noticeable, it is possible that the sheets might wrinkle inside the folded portion.
More specifically, referring to FIG. 1, the spine formation mechanism flattens a leading edge portion SA1 of a booklet SA to make a square spine SA2 shown in FIG. 2A. However, in the above-described approach, wrinkles SA3 can be created on the inner side of the booklet SA as shown in FIG. 2B.